As is well known, high-quality recording and reproduction can be achieved by applying digital techniques to signal processing of an audio signal, when recording or reproducing the audio signal.
A tone control device is known as a conventional art that processes an audio signal by digital techniques. A tone control device aims to suitably adjusting tone quality of a reproduced audio signal, depending on a program source and the state of a listening room. For this purpose, a tone control device processes an audio signal in digital form.
The tone control device can change the frequency-amplitude characteristic of an audio signal which is the subject of tone control. A change of the frequency-amplitude characteristic of an audio signal by the tone control device may cause in a change in phase of the audio signal. This changes of the assigned position in a stereoscopic reproduction sound field. For this reason, an improved tone control device is desired that causes no change in phase of an audio signal even when the frequency-amplitude characteristic thereof is varied.
A digital filter having the linear phase characteristic is attractive as a tone control device which can satisfy the above-mentioned need. A digital filter having the linear phase characteristic uses a finite impulse response (FIR) digital filter, which has a constant group delay irrespective of a change of the frequency-amplitude characteristic.
An FIR digital filter is designed to execute digital filtering operations so as to provide a desired frequency-amplitude characteristic by using a digital signal processor (hereinafter simply referred to as DSP). The number of operations (number of taps) that the DSP repetitively carries out the digital filter operation is based on a desired frequency-amplitude characteristic. It is noted that an extremely large number of operations that the DSP repetitively carries out the digital filter operation is necessary to obtain a desired frequency-amplitude characteristic which is highly precise even in a low-frequency range of the audio signal.
Therefore, such a conventional FIR digital filter needs a long operation time. The digital filter operation is repetitively carried out for predetermined times during a necessary sampling period. Therefore, a large number of multipliers and memories is necessary to execute the digital filter operation in real time.
When many multipliers and memories are formed on a single DSP chip, they will take about two-thirds of the size of the DSP chip. This needs a large chip size. On the other hand, when a DSP having a minimum of multipliers and memories is formed on a chip of small size, a real time digital filter operation needs many DSPs connected in parallel. This is not economical.
An object of the present invention is to provide an economical FIR digital filter which can process a digital audio signal in real time without decreasing economical efficiency.